Asialoglycoprotein Receptor-Mediated Gene Delivery to Hepatocytes

Aug 10, 2015 - Highly efficient, specific, and nontoxic gene delivery vector is required for gene therapy to the liver. Hepatocytes exclusively expres...
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Asialoglycoprotein Receptor-Mediated Gene Delivery to Hepatocytes Using Galactosylated Polymers Bindu Thapa,† Piyush Kumar,‡ Hongbo Zeng,† and Ravin Narain*,† †

Department of Chemical and Materials Engineering, University of Alberta, 116 Street and 85 Avenue, Edmonton, AB T6G 2G6, Canada ‡ Department of Oncology, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB Canada S Supporting Information *

ABSTRACT: Highly efficient, specific, and nontoxic gene delivery vector is required for gene therapy to the liver. Hepatocytes exclusively express asialoglycoprotein receptor (ASGPR), which can recognize and bind to galactose or N-acetylgalactosamine. Galactosylated polymers are therefore explored for targeted gene delivery to the liver. A library of safe and stable galactose-based glycopolymers that can specifically deliver genes to hepatocytes were synthesized having different architectures, compositions, and molecular weights via the reversible addition−fragmentation chain transfer process. The physical and chemical properties of these polymers have a great impact on gene delivery efficacy into hepatocytes, as such block copolymers are found to form more stable complexes with plasmid and have high gene delivery efficiency into ASGPR expressing hepatocytes. Transfection efficiency and uptake of polyplexes with these polymers decreased significantly by preincubation of hepatocytes with free asialofetuin or by adding free asialofetuin together with polyplexes into hepatocytes. The results confirmed that polyplexes with these polymers were taken up specifically by hepatocytes via ASGPR-mediated endocytosis. The results from transfection efficiency and uptake of these polymers in cells without ASGPR, such as SK Hep1 and HeLa cells, further support this mechanism. Since in vitro cytotoxicity assays prove these glycopolymers to be nontoxic, they may be useful for delivery of clinically important genes specifically to the liver.

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INTRODUCTION Success of gene therapy depends on the safe, specific, and effective delivery systems of therapeutic gene to the target cells. Recently, gene therapy using nonviral vectors has been investigated extensively because of their capacity to deliver large DNA, facile chemistry, flexible manufacturing, low immune response, and safe toxicity profiles. However, the clinical outcome is limited due to low efficiency of the carrier. A wide range of cationic polymers including poly(ethylenimine) (PEI), poly(L-lysine), and poly(amidoamines) have been synthesized and explored in gene therapy over the last few decades, and significant progress has also been reported.1 The electrostatic interaction between cationic charge of these polymers and negative charge of nucleic acid is the basis of nonviral gene delivery and it generates nanoto submicron-sized complexes. Strong cationic charge density, however, creates unnecessary cell surface disruption and cellular toxicity. In addition, nonspecific cellular interaction, poor colloidal stability, and low transfection are major hurdles that can hamper delivery capability of nonviral carriers to specific cells.1,2 Lately, significant attention has been given to the development of nonviral gene delivery carrying specific ligands targeting cell surface receptors.3,4 Receptor-mediated endocytosis is among the promising approaches to deliver therapeutic gene to specific cells and tissues types. Cell surface receptor-specific ligands, including galactose,3,5 mannose,6 folate,7 antibody,8 and asialofetuin,4 have been © 2015 American Chemical Society

conjugated onto the nonviral gene delivery vectors. Among these, asialoglycoprotein receptor (ASGPR) seemed most promising for receptor targeting gene delivery because of its high affinity and rapid internalization rate.9 Successful in vivo gene delivery specifically into hepatocytes by exploring ASGPRs therefore appears to be of great therapeutic potential since hepatocytes are responsible for the synthesis of a wide variety of proteins. Moreover, it can be a most promising tool to deliver therapeutic genes into hepatocytes to treat hepatocellular carcinoma (HCC), which is one of the most common malignancies and third leading cause of cancer-related deaths in the world. More than one million cases of HCC are reported each year, with a poor 5-year survival rate of about 7% in spite of treatment.10−12 ASGPRs are exclusively expressed in hepatoma cell lines such as HepG2 and Huh7.5 cells;13,14 therefore, a strategy of exploiting these receptors for targeted therapy may have reduced toxicity in other tissues. ASGPR selectively binds to galactose or N-acetylgalactosamine residues of deasilylated glycoproteins and then internalizes them.15 Galactose has been focused for gene and drug delivery into liver cells. We have previously reported a series of block and statistical carbohydrate-based copolymers that exhibited high gene delivery Received: July 7, 2015 Revised: August 6, 2015 Published: August 10, 2015 3008

DOI: 10.1021/acs.biomac.5b00906 Biomacromolecules 2015, 16, 3008−3020

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0.005 mmol) in DMF (1 mL). The mixture was degassed with nitrogen for 45 min, and polymerization was carried out at 70 °C for 12 h. The polymerization was then stopped by quenching in liquid nitrogen, precipitated in acetone, and extensively washed with DMF and methanol. The final product was dried and analyzed by GPC, as described above. Formulation of Polymer/DNA Complex and Characterization. Polyplexes were formed by mixing polymer with β-galactosidase plasmid in OMEM medium. Briefly, 1.2 μg of β-galactosidase plasmid was added to varying concentrations of polymer (depending on type of polymer) in OMEM medium, lightly vortexed, and incubated at 22 °C (RT) for 30 min to obtain polyplexes. Polyplexes were diluted with OMEM, and the size and surface charge of polyplexes were measured using Brookhaven Zeta Plus (zeta potential and particle size analyzer) instrument. In order to study their stability, polyplexes were diluted with complete medium and incubated at 22 °C. Then size was measured at designated time intervals. Formation of polyplexes was confirmed by agarose gel electrophoresis. Polyplexes were loaded in 0.8% agarose gel containing 1 μg/mL ethidium bromide in 1× tris acetate/EDTA buffer at 120 V for 45 min. The gel was illuminated with UV light to reveal the relative position of complexed and naked DNA using UV transilluminator (Alpha Innotech; San Leandro, CA). Biological Assessment. Cell Culture. HepG2, SK hep1, HeLa, and Huh7.5 cells were maintained in high glucose DMEM with 10% FBS and 1% penicillin/streptomycin in a humidified atmosphere in the presence of 5% CO2 at 37 °C. Cells were trypsinized with 0.25% trypsin and subcultured in tissue culture plates upon 80% confluence. Determination of Lethal Dose (LD50) Values. Toxicity of polymers was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. HepG2 cells were seeded in 96-well plates at a density of 10000 cells per well and incubated overnight in DMEM with 10% FBS and 1% antibiotic in a humidified atmosphere containing 5% CO2 at 37 °C. The media was replaced with a fresh one containing varying concentrations of polymers. After 24 h, media in each well were replaced with 100 μL of MTT reagent (1 mg/mL) and incubated at 37 °C for 2 h. The crystal formed was dissolved in 100 μL of dimethyl sulfoxide/isopropanol (1:1), and absorbance was measured at 570 nm using a Tecan Microplate reader. This experiment was done in triplicate. Untreated cells were used as positive control, and based on this, percent of cell viability was calculated. LD50 values for different polymers were calculated using Origin Pro software, and data were fit into a sigmoidal fit using the Boltzmann function. Gene Expression. HepG2, HeLa, SK Hep1 cells (100000 cells/ well), and Huh7.5 cells (150000 cells/well) were seeded in 24-well plates. Cells were allowed to adhere overnight, and then media were replaced with 10% FBS OMEM containing polyplexes formulated as described above. After 4 h of incubation, they were replaced with fresh serum containing DMEM media, and then the cells were further allowed to grow for 48 h. The cells were lysed with Triton X lysis buffer, followed by three freeze−thaw cycles. Transgene expression was determined by analyzing β-galactosidase activity.24 The transfection efficiency of each polymer was done in triplicate, mean and standard deviation were determined, and t test was performed to determine the significance. Branched PEI was used as positive control, and cells without any treatment were used as negative control. Toxicity after Transfection. HepG2 cells were seeded in 24 well tissue culture plates and treated with Polyplexes in similar ways to transfection condition as described above. After 48 h, media was replaced with 200 μL of MTT reagent (1 mg/mL) per well and incubated at 37 °C for 2 h. Medium was removed and dimethyl sulfoxide: isopropanol (1:1,v/v, 200 μL) was added to each well and the absorbance was read at 570 nm using TECAN plate reader. This experiment was performed in triplicate. The untreated cells were used as positive control to calculate the percent of cell viability. Fluorescent Labeling of Plasmid. Gwiz plasmid was labeled according to the manufacturer protocol. Plasmid (1 μg) was incubated with DNA labeling buffer and Cy3 fluorescent dye at 37 °C for 2 h, followed by purification. Uptake of Polyplexes Using Confocal Microscopy and Flow Cytometry. Cellular uptake of polyplexes was analyzed using confocal

efficacy with lower toxicity than conventional transfection agents.16−18 Sizes, compositions, and architectures of these cationic carbohydrate copolymers were found to impact DNA condensation and transfection efficiency.16−18 Use of carbohydrates and their cationic analogues for gene delivery is a natureinspired approach to produce biocompatible gene delivery vectors. Carbohydrate residues can facilitate the condensation of DNA via hydrogen bonding, thus, reducing the need for excess cationic charge and eventually decreasing the toxicity of the vector system.19,20 In this study, we have synthesized a series of galactose containing block and statistical cationic copolymers of different sizes and compositions via reversible addition− fragmentation chain transfer polymerization (RAFT) technique. Subsequently, their transfection efficiencies were evaluated in ASGPR expressing and nonexpressing hepatocytes.

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MATERIALS AND METHODS

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EXPERIMENTAL SECTION

Materials. O-Nitrophenyl β-D-galactopyranoside, (ONPG; enzymatic), asialofetuin, formalin, β-mercaptoethanol, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), hyperbranched polyethylenimine (PEI; Mw = 25 kDa), and 4,4-azobis(4-cyanovaleric acid) (ACVA)were purchased from Sigma-Aldrich. Cell culture media Dulbecco’s Modified Eagle Medium (DMEM; high glucose with glutamine and sodium pyruvate), Opti-MEM (OMEM), penicillin (10000 U/mL), streptomycin (10 mg/mL), 0.25% trypsin, and fetal bovine serum (FBS) were obtained from Invitrogen. Micro BCA assay kit was obtained from Fisher Scientific. Gwizβ-galactosidaseplasmid was purchased from Aldevron. Cy3 plasmid labeling dye was purchased from Bio Mirus. 4-Cyanopentanoic acid dithiobenzoate(CTP), 2-aminoethyl methacrylamide (AEMA), and 2-lactobionamidoethyl methacrylamide (LAEMA) were synthesized according to a previously reported procedure.21−23 All other chemicals used were of reagent grade.

Polymer Synthesis and Modifications. The polymers of varying degree and architecture were synthesized via RAFT polymerization according to previously reported protocols.24 The statistical copolymers of AEMA and LAEMA were prepared at 70 °C in the presence of initiator 4,4′-azobis(4-cyanovaleric acid) (ACVA) and chain transfer agent 4-cyanopentanoic acid dithiobenzoate (CTP). Briefly, for the polymer of targeted Mn = 30 kDa and DPn = 100, AEMA (0.130 g, 0.78 mmol) and LAEMA (0.370 g, 0.79 mmol) were dissolved in 5 mL of double distilled water, followed by addition of CTP (3.99 mg, 0.014 mmol) and ACVA (1.995 mg, 0.0071 mmol) in 1 mL of N,N′dimethylformamide (DMF). Then the mixture was degassed with nitrogen gas for 45 min and maintained at 70 °C for 12 h under constant stirring. The polymerization was quenched with liquid nitrogen and precipitated in acetone, followed by washing with methanol and DMF. The structural composition was analyzed by 1H NMR spectroscopy (Varian 500 NMR). Molecular weight and molecular weight distribution of the polymers were determined by GPC (Viscotek GPC system) using 0.5 M sodium acetate/0.5 M acetic acid buffer as eluent at a flow rate of 1.0 mL/min at 22 °C. Pullulan standards (Mw = 6.2−113 kDa) were used for calibration. For the diblock copolymerization, macroCTA of AEMA was prepared and then used in the polymerization of LAEMA. In a typical reaction of macroCTA of AEMA (Mn = 20 kDa), AEMA (0.5 g, 3.02 mmol) was dissolved in double distilled water (4 mL), followed by addition of CTP (7.049 mg, 0.025 mmol) and ACVA (3.52 mg, 0.013 mmol) in 1,4-dioxane (2 mL). The mixture was degassed with nitrogen gas for 45 min and placed in an oil bath at 70 °C with constant stirring for 6 h. The polymerization was quenched with liquid nitrogen. Polymer was precipitated in acetone, extensively washed with methanol, and dried. Molecular weight and molecular weight distribution were determined by GPC, as described above. The obtained macroCTA (0.2 g, 0.01 mmol) and LAEMA (0.201g, 0.43 mmol) were dissolved in double distilled water (5 mL), followed by addition of ACVA (1.4 mg, 3009

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Scheme 1. Synthesis of Statistical Cationic Glycopolymer Using 4-Cyanopentanoic Acid Dithiobenzoate (CTP) as Chain Transfer Agent and 4,4′-Azobis(4-cyanovaleric acid) (ACVA) as Initiator via the RAFT Polymerization Technique

microscopy and flow cytometry involving Cy3-labeled DNA. HepG2 cells were seeded in six-well tissue culture plates containing glass coverslips overnight. Upon 80% of confluency, media was replaced by OMEM medium containing polyplexes with Cy3-labeled plasmid. Media was removed after 4 h of incubation and washed with PBS buffer at a pH 7.4 thrice, followed by fixation using 3.7% formalin in PBS (pH 7.4). The cells containing slides were fixed on microscope slide and analyzed using a Fluoview FV 10i Olympus confocal microscope. Samples were excited at 550 nm and detected at 570 nm. For the flow cytometry studies, HepG2 and SK Hep 1 cells were seeded in 24-well plate (100000 cells per well) and allowed to adhere overnight. The media was replaced with a media containing polyplexes prepared with Cy3-labeled DNA, and incubated for 4 h. The cells were rinsed (3×) with PBS, trypsinized, and fixed with 3.7% formalin. The uptake of polyplexes was quantified using FACS Calibur MACPro acquisition flow cytometer using FL-2 channel. The untreated cells were used as negative controls. Asialofetuin (ASF) Competition Assay. HepG2 cells were seeded onto 24-well plates (100000 cells/well) and allowed to adhere overnight. Cells were incubated with 500 μL of 1 mg/mL of asialofetuin in fresh media. After 1 h, media was replaced with the media containing polyplexes and incubated. In another group, cells were treated with polyplexes along with 1 mg/mL asialofetuin. After 4 h of incubation, media was replaced with the fresh DMEM media and the cells were allowed to grow for 48 h. β-Galactosidase expressed was determined as described above.

A library of galactose containing cationic glycopolymers was synthesized via RAFT polymerization. Schemes 1and 2 illustrate the synthesis of block and statistical cationic glycopolymers. RAFT polymerization approach allows the synthesis of various cationic polymers of controlled dimensions and architecture in the absence of metal catalysis.24,30,31 Several linear and hyperbranched cationic copolymers of block and statistical architecture were synthesized by RAFT, and their gene delivery efficacy was evaluated.24,29 A series of cationic homopolymer and copolymer with block and statistical architecture of varying molecular weights and compositions were synthesized by RAFT using 2-lactobionamidoethyl methacrylamide (LAEMA) and 2-aminoethylmethacrylamide (AEMA) as monomers (Schemes 1 and 2). Statistical copolymers of AEMA and LAEMA were synthesized by RAFT polymerization in the presence of CTP and ACVA (Scheme 1). For block copolymer, LAEMA was polymerized in the presence of P(AEMA) macroCTA and ACVA by RAFT polymerization (Scheme 2). The resulting statistical and block polymers were characterized by gel permeation chromatography (GPC) for molecular weights, polydispersities (PDI), and molecular weight was revealed to be between 6.8 and 41 kDa (Table 1). For comparison, homopolymer of AEMA (Mn = 15 kDa) was synthesized. Furthermore, compositions of polymers were calculated by 1H NMR spectroscopy. Figure S2 is a representative of 1 H NMR spectra of statistical copolymer. In order to investigate the role of polymer size and carbohydrate segment in gene delivery and masking toxicity, molecular weight and carbohydrate content were varied in both statistical and block copolymers (Table 1). The resulting polymers were studied for their DNA complexation and gene delivery efficacy. Toxicity of polymer plays a crucial role in their use as gene carrier in vivo. Toxicity of polymers were evaluated by using MTT assay in HepG2 cells and lethal dose 50 (LD50) was determined (Table 1). Toxicity of polymers was found to be dependent on the architecture and carbohydrate content of polymers, which is in accordance with previous reports.24,32 As expected, homopolymer, P(AEMA)90 is most toxic, and the toxicity decreases as carbohydrate content of polymer increases. Incorporation of carbohydrate residues in the copolymer decreased the toxicity significantly and copolymer with LD50 more than 1470 μM was achieved. However, higher molecular weight polymers were found to be more toxic than lower molecular weight polymers despite the presence of carbohydrate moieties. LD50 of polymer P(AEMA74-st-LAEMA62) of Mn −41 kDa was found to be higher than P(AEMA81-st-LAEMA58) of same molecular weight

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RESULTS AND DISCUSSION Use of natural cationic polysaccharides as gene delivery vehicles is hindered by their polydisperse nature, and the gene delivery efficacy is dependent on their architecture, size, and composition. Therefore, a facile approach to synthesize cationic glycopolymers of well-defined size and flexible architecture (block vs random polymers) with narrow polydispersities is necessary. Recent advances in the field of polymer chemistry have made it possible to produce well-defined and nontoxic cationic polymers with desired molecular weights and compositions, which allow a clearer improved understanding of structure−functionality relationships of nonviral gene delivery vehicles.20,25−28 Careful engineering of materials has therefore enabled us to overcome some of the major drawbacks of previously studied vectors leading to major advances in the field of therapeutic gene delivery.24,9 Our previous studies demonstrated that gene delivery efficacy, toxicity and DNA binding properties of polymers were affected by the parameters, including composition, architecture, and molecular weight.16,24,29 In addition, composition of carbohydrate segment was critical, which determines the gene delivery efficacy and toxicity of polymer.24 3010

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Scheme 2. Synthesis of Cationic Block Glycopolymer Using 4-Cyanopentanoic Acid Dithiobenzoate (CTP) as Chain Transfer Agent and 4,4′-Azobis(4-cyanovaleric acid) (ACVA) as Initiator via RAFT Polymerization Technique

also showed increase in toxicity with increasing cationic charge density of β-cyclodextrin conjugated poly(amidines) polymers.33 Furthermore, block copolymers are not that capable to mask the cationic component, which makes them more toxic. This leads to valuable information that toxicity of polymer depends not only on the type and composition of monomers and molecular weight, but also on the architecture of polymers. As shown in Table 1, all resulting cationic glycopolymers were less toxic than polyethylenimine (PEI; Mw = 25 kDa), which has a LD50 value of 2.334 ± 0.57 μM in HepG2. In spite of demonstrating high transfection efficiency, use of PEI as nonviral gene delivery vehicles was compromised due to its toxicity. Hence, resulting galactose based glycopolymers were safer than PEI for in vivo use as gene delivery agents. The polymers produced were complexed with the β-galactosidase plasmid for 30 min at 22 °C, and the sizes and charges of the resulting complexes were determined. Binding of polymer with plasmid was characterized by gel electrophoresis shift assay (Figure 1). Although gel electrophoresis shift assay showed formation of complexes at very low polymer to plasmid ratio, higher ratios were used to obtain stable and nanometer sized particles which are essential criteria for high transfection efficiency. N/P ratios of the polyplexes used for transfection are listed in Supporting Information, Table S1. Net surface charge and size of polyplexes at polymer/plasmid ratio (showing optimum gene expression) were measured. Surface charge of polyplexes varies as a function of sizes, compositions and architectures of polymers. As expected, the surface charge is the highest for the cationic homopolymer based polyplexes and is lower for the copolymer based polyplexes

Table 1. Molecular Weight Distribution of Polymers and Their Lethal Dose 50 (LD50) Obtained by MTT Assay in HepG2 Cells polymer composition Homopolymer P(AEMA90) Statistical Copolymers P(AEMA10-st-LAEMA11) P(AEMA22-st-LAEMA22) P(AEMA43-st-LAEMA42) P(AEMA74-st-LAEMA62) P(AEMA81-st-LAEMA58) Block Copolymers P(AEMA17-b-LAEMA17) P(AEMA42-b-LAEMA48) P(AEMA58-b-LAEMA56) PEIa

GPC Mn (kDa)

Mw/Mn

LD50 (μM)

15

1.3

17.12 ± 1.67

6.8 14 26 41 41

1.2 1.2 1.5 1.3 1.3

>1470 >714 >384 172 ± 14 56.87 ± 8.29

11 30 36

1.5 1.3 1.4

68.85 ± 3.19 94.65 ± 11 84.52 ± 7.094 2.334 ± 0.57

Commercially available branched polyethylenimine of Mw ∼ 25 kDa. Values are mean ± SD (n = 3).

a

(172 ± 14 vs 56.87 ± 8.29, respectively) which conforms decrease in toxicity with increasing carbohydrate content. Block copolymers were found to be more toxic than their corresponding statistical copolymers. For example, block copolymer P(AEMA42-b-LAEMA48) with LD50 of 94.65 ± 11 μM is more toxic than P(AEMA43-st-LAEMA42) with LD50 of more than 384 μM in spite of higher content of carbohydrate in first one. Similar trends were observed in our previous study.24 Increase in the LAEMA content is expected to decrease the net positive charge of the cationic polymer, thus reducing the toxicity. Huang et al. 3011

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Figure 1. Agarose gel electrophoresis showing complexation of polymer with β-galactosidase plasmid (top). Dynamic light scattering (DLS) and zeta potential data for the glycopolymer-β-galactosidase plasmid polyplexes (bottom). All samples were at fixed plasmid concentration, while polymer concentrations were varied to obtain stable particles. Polymer to plasmid ratios were similar to that used for transfection.

Figure 2. DLS analysis of the time dependent aggregation of polyplexes in media.

content on the shell, contributing to a reduction in net positive charge on the polyplexes surface. Therefore, zeta potentials of block copolymer based polyplexes were found to be close to zero. In contrast, for polyplexes made with statistical copolymers, there is an equal possibility of displaying carbohydrate and cationic segment on the surface displaying net positive charge on the surfaces.

(Figure 1). Net surface charge of statistical copolymer-based polyplexes is higher than that of the block copolymer-based polyplexes, which proves the higher binding efficiency of the block copolymers and is in accordance with a previous study.24 In addition, polyplexes with block copolymers contain most of the condensed DNA in the core displaying higher carbohydrate 3012

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Figure 3. Transfection efficacies of polymers in HepG2 cells in the presence of 10% FBS evaluated by β-galactosidase assay. DNA amount is fixed to 1.2 μg in all experiments. Each value represents mean ± SD (n = 3).

Figure 4. Transfection efficacies of polymers in Huh7.5 cells in the presence of serum evaluated by β-galactosidase assay. DNA amount is fixed to 1.2 μg per well of 24-well plate in all experiments. Each value represents mean ± SD (n = 3).

polyplexes with block copolymer P(AEMA58-b-LAEMA56) were very stable, and the size did not change over time (Figure 2). Size of polyplexes with all statistical copolymers increased after incubation in media at 22 °C for 2 h, whereas polyplexes with block copolymer remained stable under similar conditions (Supporting Information, Figure S1). Higher positive surface charge of statistical copolymer resulted in aggregation of polyplexes. For the block copolymers, the dense carbohydrate shells seem to play a major role in the stability of those polyplexes. Modification of cationic polymers with various biocompatible moieties without affecting gene delivery efficacy has been reported.34−38 For example, derivatization with polyethylene glycol (PEGylation) is one the most common approaches for this purpose. PEGylation of cationic polymers is known to solve the

Polyplexes with different polymers showed a size between 100 and 400 nm with net positive zeta potential values (Figure 1). In general, size of polyplexes decreases with increasing molecular weight of polymers, and the size of block copolymer-based polyplexes were found to be smaller than their corresponding statistical copolymers (Figure 1). These results are in agreement with the previous report where effects of carbohydrate compositions and architectures of copolymers on size, charge, and transfection efficiencies were studied.24 As compared to statistical copolymers, block copolymers may condense DNA more efficiently in their core resulting in smaller size complexes. In addition, these polyplexes were much more stable than polyplexes derived from statistical copolymers. Size of polyplexes obtained from statistical copolymer, P(AEMA81-st-LAEMA58) and PEI increased up to micron size with time. However, 3013

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Scheme 3. Complexation of β-Galactosidase Plasmid with Galactose-Based Statistical and Block Copolymers, with Block Copolymer-Based Polyplexes Displaying Higher Surface Galactose Residues Accounting for the Enhanced Uptake in Asialoglycoprotein-Rich Liver Cells

problem of cytotoxicity, aggregation, and nonspecific protein binding in vivo.35,39 Several carbohydrates, such as β-cyclodextrin and chitosan, have been used for gene delivery purposes.34 Furthermore, various polysaccharide backbones were modified with different cationic polymers to produce less toxic carbohydrate-based gene delivery vehicles with enhanced transfection efficiencies. Modifying backbones of arabinogalactan, dextran, and pullulan with PEI, spermine, or other cationic polymers has demonstrated that charge ratio and the nature of carbohydrate moiety play a major role in gene delivery efficacy. Linear and branched PEIs grafted with β-cyclodextrin showed reduced transfection efficiency and lower cytotoxicity along with increasing grafting density.40 In addition to lower toxicity of cationic polymer, carbohydrate molecules can also be used as ligands for specific receptors. Lactose and galactose, for instance, were conjugated to the polymeric carrier for targeting specifically ASGPR of hepatocytes.41,42 There is still need to discuss the effects of architecture and composition of carbohydrate on ASGPR-mediated gene delivery into hepatocytes. The resulting polymers contain galactose as a pendant making it accessible for interaction with ASGPR of hepatocytes even when complexed with DNA. Hence gene delivery efficacy of these polymers was determined using β-galactosidase plasmid in ASGPR-rich (HepG2 and Huh7.5 cells) and ASGPR-deficient (SK hep1 and HeLa cells) cells.

ASGPR is a c-type lectin whose main function is to remove desialylated serum proteins such as fibronectin.43 ASGPR has carbohydrate recognition domain, which can bind at least a single-terminal galactose or galactosamine residues and binding affinity increases with valence of sugar residues via phenomenon called cluster effect.44 In both HepG2 and Huh7.5 cells, β-galactosidase plasmid transfection was found to be dependent on sizes, architectures, and compositions of polymers (Figures 3 and 4). Homopolymer, P(AEMA)90, demonstrated the highest transfection, which is equal or comparable to branched PEI (Mw = 25 kDa) at given plasmid dose. It should be noted that P(AEMA)90 had higher LD50 values than branched PEI, however, it remained toxic to cells. Incorporation of carbohydrate units greatly decreased toxicity as well as transfection. As reported in a previous study,24 transfection efficiency of higher molecular weight polymer was higher than the corresponding lower molecular weight counterpart. For statistical copolymer, transfection efficiency is enhanced with increase in the cationic moieties. For example, P(AEMA81-st-LAEMA58) expressed higher transfection than P(AEMA74-st-LAEMA62), although both of them have same molar masses. Similarly, block copolymer of higher molecular weight, P(AEMA58-b-LAEMA56) had high transfection efficiency than low molecular weight polymer P(AEMA17-b-LAEMA17). Like statistical copolymer, block copolymer having more cationic moiety P(AEMA58-b-LAEMA56) gave high transfection efficiency than copolymer having less cationic moiety P(AEMA42-b-LAEMA48), 3014

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Figure 5. Transfection efficacies of polymers in HeLa (A) and SK hep 1 (B) cells evaluated by β-galactosidase assay. DNA amount is fixed to 1.2 μg in all experiments. Each value represents mean ± SD (n = 3).

Figure 6. Effects of the presence of asialofetuin on transfection efficacy of polymers in their optimal polymer/DNA ratio in HepG2 cells. Asialofetuin was added to HepG2 cells 1 h prior treatment with polyplexes or together with polyplexes. DNA amount is fixed to 1.2 μg in all experiments. Each values represents mean ± SD (n = 3). Statistical analysis was performed by analysis of variance. *(p < 0.003) and **(p < 0.0005) indicate significant difference when compared to polyplexes only.

although the molecular weights are very close and the polyplexes are similar in terms of size and charge. These results emphasize the importance of appropriate molecular weight and ratio between cationic and glucose monomer for efficient transfection. Block copolymer, P(AEMA58-b-LAEMA56), has higher transfection efficiency than the corresponding statistical copolymer of similar masses. It should be noted that block copolymer P(AEMA58-bLAEMA56) showed greater transfection efficiency, although polyplexes with plasmid had surface charge close to zero. Both HepG2 and Huh7.5 cells are hepatocytes expressing ASGPR on their surface. Complexes with block copolymer contain galactose on outer shell that facilitates ASGPR mediated uptake of these particles. ASGPR can recognize and internalize the galactose terminal through receptor-mediated endocytosis (see Scheme 3). ASGPR are present only on the surface of mammalian hepatocytes.

Furthermore, transfection efficiency of both statistical and block copolymers in HeLa cells and SK hep1 cells was negligible, whereas PEI and AEMA90 still exhibited high transfection in those cell lines (Figure 5). It is worth mentioning that HeLa and SK hep1 cells do not express ASGPRs on their surface. Although SK Hep 1 is hepatocytes, ASGPR is absent on their surface and HeLa cell is derived from cervical cancer. Gene delivery efficacy of all polymers in HeLa cell is lower than in HepG2 cells. Although PEI and P(AEMA)90 showed some transfection, the efficiencies of block and statistical copolymers were insignificant. Uptake of polyplexes with PEI and P(AEMA)90 may be driven by electrostatic interaction between polyplexes and cell membrane; therefore, transfection efficiency is not hepatocytespecific. However, transfection efficiencies of block and statistical copolymers were negligible in HeLa cells which confirm that 3015

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Figure 7. Study of polyplexes uptake in the presence and absence of asialofetuin using confocal microscope in HepG2 cells. Upper images represents the uptake of Cy3-labeled plasmid using glycosylated block copolymer P(AEMA58-b-LAEMA56) and lower images using P(AEMA)90 in the absence and presence of asialofetuin.

with polyplexes. Transfection efficiency of PEI and homopolymer remains same regardless of the presence of asialofetuin. In the case of block copolymer P(AEMA58-b-LAEMA56), transfection efficiency was greatly reduced in the presence of asialofetuin (Figure 6). Decrease in transfection is more pronounced when complexes and asialofetuin were treated together. This may be due to the high competition between asialofetuin and polyplexes toward ASGPR. In case of 1 h pretreated with asialofetuin, ASGPR may internalize a portion of asialofetuin, followed by recycle of ASGPR on surface of HepG2 cells. Nevertheless, a significance portion of ASGPR may be blocked by asialofetuin, resulting in reduced transfection. However, asialofetuin treatment did not inhibit the transfection completely, which may be due to higher avidity of multiple galactose molecules on the surface to polyplexes. Lee et al. has demonstrated that binding affinity to ASGPR can be increased by multiple galactose molecules by so-called cluster effect in which distance between galactose residues must be 15−25 Å.44 In order to achieve gene expression, several barriers, notably cellular uptake, endosomal escape, and nuclear uptake, must be overcome, as any of these can significantly impact the transfection efficacy. In our design, uptake of polyplexes is mediated by the ASGPR, which recognizes galactose residues of the copolymers and internalize them via receptor mediated endocytosis. Hence, changes in amount of ASGPR on cell surface will greatly affect the transfection efficiency. Here, we investigated the uptake of polyplexes containing Cy3-labeled plasmid using confocal microscopy and flow cytometer in the presence and

gene delivery efficacy of galactose containing statistical and block copolymer is hepatocytes specific. This observation is in support of finding by Wang et al., where glycosylated poly(ethylene glycol) derivative-graf t-polyethylenimine exhibited reduced transfection in HeLa cells as compared to HepG2 cells.45 Importantly, transfection efficiency of block and statistical copolymer was significantly reduced while efficiency of PEI and P(AEMA)90 remains intact in SK Hep 1 cells. This proves ASGPR specificity of block and statistical copolymer for gene delivery. Polyplexes with P(AEMA81-st-LAEMA58) showed transfection in SK hep 1 cells to some extent, which may be driven by its net positive charge. Gene delivery efficiency of P(AEMA58-b-LAEMA56), which was highest among glycopolymer in both ASGPR-expressing hepatocytes HepG2 and Huh7.5 cells, was insignificant in ASGPR-deficient hepatocyte SK hep 1 cells. Since net surface charge of polyplexes with P(AEMA58-b-LAEMA56) is near to neutral, electrostatic interaction may not be enough for uptake and ASGPR-mediated endocytosis is not available in SK hep 1 cells. Hence, P(AEMA58b-LAEMA56) was unable to transfect SK hep 1 cells. These results suggested galactose containing copolymer promotes specific delivery of genes into hepatocytes, which express ASGPR on surface. To further confirm that galactose-based polyplexes were recognized by ASGPR of HepG2 cells, competitive assay of transfection and uptake of polyplexes in the presence of free asialofetuin was performed. Recognition and binding to ASGPR facilitate the receptor mediated endocytosis. HepG2 cells were pretreated with asialofetuin for 1 h prior to treatment or together 3016

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Biomacromolecules absence of asialofetuin. HepG2 cells were first treated with asialofetuin for 1 h, followed by the addition of the polyplexes. Homopolymer P(AEMA)90 showed a similar extent of uptake, regardless of pretreatment with asialofetuin. As expected, uptake of polyplexes with block copolymer P(AEMA58-b-LAEMA56) decreased significantly in the presence of asialofetuin (Figure 7). ASGPR-mediated uptake of polyplexes with galactosylated copolymers in HepG2 and SK Hep 1 cells was further confirmed by flow cytometry. Uptake of Cy3-labeled plasmid using homopolymer, statistical, and block copolymers were studied in the presence and absence of asialofetuin in SK Hep1 and HepG2 cells. Flow cytometer analysis of percentage of positive cells supports observation that asialofetuin competitively inhibit uptake of polyplexes with galactose containing copolymers which supports observation from confocal microscopy images (Figure 7). HepG2 cells were treated with free asialofetuin for 1 h followed by treatment with polyplexes of Cy3-labeled plasmid and polymers. Uptake of polyplexes was evaluated in terms of percentage of positive cell population by flow cytometry in FL2 channel. Positive cell population percentage was significantly reduced when treated with polyplexes with P(AEMA58-bLAEMA56) and P(AEMA43-st-LAEMA42 in the presence of asialofetuin (Figure 8). On the other hand, asialofetuin treatment did not affect positive cell population when using PEI and homopolymer AEMA90. Hence, uptake of polyplexes was found to be dependent on the ASGPR located on the surface of hepatocytes, which enables hepatocytes-specific targeting features in these copolymers. In HepG2 cells, although uptake of P(AEMA43-st-LAEMA42) is higher than of P(AEMA58-bLAEMA56), transfection efficiency is low. Although cellular uptake of polyplexes is first step, it may not be enough for successful transfection. Fate of internalized polyplexes will depend on their intracellular routing which determines transfection efficiency. Route of internalization depends on the type of delivery systems. In general, complexes bound to cell surface receptor enter cells via clathrin mediated endocytosis and cationic complexes without any targeting ligands is guided through electrostatic interaction with anionic cell surface, proteoglycans, and pulled into cells via phagocytosis.2,46 According to Goncalvel et al., although polymer and plasmid polyplexes enter into HepG2 cells via both clathrin dependent and independent pathways, clathrin dependent pathways are most productive for efficient gene transfection in HepG2 cells.47 Surface charge of polyplexes with block copolymer P(AEMA58-bLAEMA56) is close to neutral, whereas with statistical copolymer P(AEMA43-st-LAEMA42) is positive. So uptake of polyplexes with P(AEMA58-b-LAEMA56) is mainly via ASGPR-mediated clathrin dependent pathways. Besides ASGPR-mediated endocytosis, polyplexes with statistical copolymer can enter the cell via nonspecific manner because of its net positive charge. Since clathrin dependent pathway is most effective for gene transfection in HepG2 cells, P(AEMA43-st-LAEMA42) have lower transfection than P(AEMA58-b-LAEMA56) in spite of higher uptake. Another reason for lower transfection efficiency of P(AEMA43-st-LAEMA42), in spite of high uptake, may be due to its limited escape from endosomes. Nevertheless, precise role of architecture of polymer in gene transfection requires further investigation. Moreover, uptake of complexes with statistical and block copolymer was negligible as compared to homopolymer, P(AEMA)90, and PEI in SK Hep 1 cells (Figure 9), confirming the specificity of these copolymer toward ASGPR-expressing hepatocytes. Hence, inability of block and statistical copolymer for transfection in SK Hep 1 cells (Figure 5) is because of

Figure 8. Uptake of polyplexes in the presence and absence of asialofetuin using flow cytometer in HepG2 cells. For a competitive assay, cells were incubated with asialofetuin for 1 h prior to adding polyplexes. Each value represents mean ± SD (n = 3).

insufficient uptake (Figure 9). These findings suggested that galactose moieties of the polymer are exposed on the surface of the polyplexes, irrespective of the complexation with DNA, and can bind to the ASGPR of hepatocytes. This result also supports reduction of transfection in the presence of asialofetuin (Figure 6) and confocal microscope image (Figure 7). Several galactose-modified cationic lipids and polymers as nonviral vehicles for delivering hepatocytes-specific gene have been developed. ASGPR-mediated endocytosis is central to the specificity of these vehicles toward hepatocytes. ASGPR is a c-type lectin whose main role is clearance of desialylated serum proteins, such as fibronectin43 and all IgA2 allotypes.48 ASGPR has carbohydrate recognition domain that can bind galactose or galactosamine residues, and affinity of ligands increases with the valence of sugar residues due to so-called “cluster effect”.44,49 Cluster effect depends on the distance between galactose residues and the optimum distance was 15−25 Å.44,50 In this study, galactose containing block copolymer P(AEMA58-b-LAEMA56) 3017

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Figure 9. Flow cytometry analysis of uptake of polyplexes in SK hep 1. Cells were treated with the polyplexes of Cy3-labeled plasmid and polymers and uptake was quantitatively evaluated Histogram of SK hep 1 cells (top) and percentage of positive population (bottom) after incubation with polyplexes. Each value represents mean ± SD (n = 3).

Figure 10. Post-transfection toxicity of polymer-based polyplexes at various polymers to plasmid ratios as measured by MTT assay in HepG2 cells. Polymer to plasmid ratio and plasmid amount used for each polymer is similar to that used for transfection.

showed high transfection efficiency into hepatoma cells (Figures 3 and 4). Moreover, observations of cellular uptake and effects of ASGPR on uptake (Figures 7 and 8) indicate that the galactose residues are exposed on the surface of polyplexes, and the distances between residues are optimal.

Post-transfection toxicity assays of polyplexes revealed that PEI and cationic homopolymer exhibits strong cytotoxicity due to their higher positive charge density on their surfaces. However, both statistical and block glycopolymers were found to be nontoxic, exhibiting more than 80% cell viability (Figure 10). 3018

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Considering cytotoxicity, hepatocellular specificity and transfection efficiency, these galactose-based block copolymers are superior for gene delivery in hepatocytes.

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CONCLUSIONS We have synthesized novel cationic galactosylated glycopolymer for specific gene delivery to hepatocytes. In both HepG2 and Huh7.5 cells, galactose containing block copolymer P(AEMA58b-LAEMA56) showed the highest transfection efficiency, which is driven by the strong interactions of the galactose residues of the polyplexes with the overexpressed ASGPR of hepatocytes. The block copolymer architectures ensure a relatively high density of galactose residues on the surface in addition to increasing the stability of the polyplexes and, hence, making the system highly effective for gene delivery to hepatocytes. Due to the high stability of those block copolymer-based systems, they are suitable for in vivo application. Attenuation in transfection and uptake of galactose containing copolymer in the presence of asialofetuin confirm the hepatocytes selectivity of polymer. In addition, transfection efficiency of these polymers in ASGPR deficient cells is negligible. Finally, these glycopolymers are nontoxic and therefore are attractive for liver targeting gene delivery system. In vivo gene delivery efficacy of these polymers into liver is planned.

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ASSOCIATED CONTENT

S Supporting Information *

DLS analysis of aggregation of polyplexes in medium and N/P ratio of polyplexes of each polymer. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.biomac.5b00906. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 780-492-1736. Fax: 780-4922881. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from Alberta Innovates Health Solutions CRIO Program grant (AIHS 201201164, P.K.), and NSERC CREATE cMIPP grant (P.K.), the Natural Sciences and Engineering Research Council of Canada (NSERC), Discovery Grant program grant (R.N., H.Z.), and Canadian Foundation for Innovation (CFI). Yinan Wang is thanked for synthesizing some of the polymers used in this work.

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REFERENCES

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